Many systems are known in the prior art for validating documents and, particularly, for validating pieces of currency either to permit change to be given or to establish a credit for the sale of goods or services. In some of these systems of the prior art, light from at least one source may be directed toward the document to be validated and either reflected or transmitted light or both may be sensed by suitable detectors to determine an optical characteristic of the document under test. While these systems operate in a generally satisfactory manner, their reliability may be adversely affected by variations in light source intensity and detector sensitivity such that they may reject what is a genuine document, while at the same time accepting a bogus document.
Variations in light source intensity and detector sensitivity may, for example, be caused by gradual degradation of intensity and sensitivity over the lifetime of the source and detector, variations in the light source supply voltage and the effects of changes in ambient temperature. Many document validators are adapted to compensate for variations in light source intensity, such for example as those shown in Gorgone et al U.S. Pat. No. 4,147,430, Iannadrea et al U.S. Pat. No. 4,183,665, Gorgone et al U.S. Pat. No. 4,127,328 and Haville U.S. Pat. No. 3,340,978. Each of the above systems, however, utilize a separate light detector for monitoring the light source and none compensate for variations in the sensitivity of the light detectors used for validation purposes.
Williams U.S. Pat. No. 4,255,057, issued Mar. 10, 1981, describes a system for determining the quality of currency in which a bill is illuminated by a light source and a pair of detectors and converters provide a first voltage output proportional to the amount of light transmitted through the bill and a second voltage output proportional to the amount of light reflected from the bill. Variations in the light source are compensated for by the use of a peak detector circuit which holds a voltage measured by the transmission detector when no bill is in place. The first and second voltage outputs are then divided by the voltage output of the peak detector and used to obtain a voltage proportional to the light absorbed by the bill independent of variations in light source intensity. This "absorption" voltage determines the quality of the bill.
While the system disclosed in Williams compensates for variations in light source intensity without the use of separate detectors, it does not compensate for variations in the sensitivity of both the reflection and the transmission detectors nor does it provide a system for monitoring the intensity of more than one light source.